The present invention relates generally to radar systems and more particularly to a radar systems and method for mapping surface currents in large bodies of water.
Low-frequency backscatter radar systerns, operating in the MP, HF, VHF, and low UHF bands are widely used for mapping surface water velocity such as currents on the oceans and flow along rivers.
The most commonly used are backscatter radar systems that use Doppler principles to measure current velocity. Two effects contribute to the backscatter Doppler shift: (i) the motions of waves that Bragg scatter, i.e., whose lengths are exactly half the radar wavelength; and (ii) the component of the underlying current that transports the surface waves along the bearing of the radar. For a backscatter radar, the wave Doppler occurs at a single frequency and is precisely known, and can be readily subtracted from a measured Doppler shift to get the desired radial current speed. Thus, the waves serve as tracers of the underlying currents. That is, information about these waves is not being sought; it is only necessary that they be present to serve as radar targets.
Range or distance to the scattering cell is obtained from the time delay between transmit and received radar signals as described, for example, in commonly assigned U.S. Pat. No. 5,361,072, which is incorporated herein by reference. Following range processing, the echo time series for each range cell is Fourier transformed to obtain Doppler spectra and cross spectra among several receive antennas or elements. The Bragg peaks used for current extraction are identified and isolated. At each Doppler spectral bin (corresponding to a definable current radial velocity), the angular bearing of the echo is obtained from the directional properties of the receive antennas using a bearing determining algorithm. One suitable bearing determining algorithm, for example, is the MUltiple SIgnal Classification (MUSIC) direction-finding algorithm as described in commonly assigned U.S. Pat. No. 5,990,834, which is incorporated herein by reference. Thus, the backscatter radar makes its measurements in a polar coordinate system in which radial current velocity is mapped as a function of distance and bearing angle.
A single backscatter radar system can measure only the radial component of a two-dimensional horizontal current velocity vector. Therefore, normally two backscatter radar systems are used in pairs, spaced tens of kilometers apart along the coast and operating independently. Based on the known geometry and location of a mutually observed scattering cell two resulting radial velocity components are combined to produce a total velocity vector map across the overlapping coverage zone. Thus, one shortcoming of conventional systems for mapping surface water velocities is the need for multiple, costly backscatter radar systems.
Another shortcoming with conventional systems is that backscatter radar systems cannot resolve a total vector on or near a line joining the two backscatter radar systems, because both are measuring the same component of velocity. This is particularly a problem for regions falling along the coast or across rivers and mouths of bays, which are often of great interest.
An alternative approach for mapping surface water velocities uses a bistatic radar system in which one or more receivers operate with two or more transmitters spaced apart therefrom. Resultant echo sets based on the signals simultaneously radiated from each isolated transmitter-receiver pair, can then be used to construct non-parallel components of velocity at the scattering cell. A major drawback to this approach is the expense of synchronizing a geographically separated transmitter and receiver. Previous bistatic systems have employed highly stable Cesium or Rubidium timing standards, or oven-controlled phase-lock loops to maintain coherent signal bases at the separated points. In addition, to being expensive such methods are ill suited to harsh environments to which ocean sensing radar systems are routinely exposed.
A more fundamental problem with the use of bistatic radar systems is that in contrast to backscatter radar systems, contours of constant time delay for bistatic radar systems define ellipses confocal about the transmitter and receiver pair. As a result, the Doppler shift due to the wave motion is no longer a constant as it is with backscatter radar systems, but varies with position around the cell or contours of constant time delay.
Accordingly, there is a need for a radar system and method of operating the same that improves coverage area and accuracy in regions where geometry of the system would normally preclude monitoring with conventional backscatter radar systems. It is also desirable that the radar system and method be robust and inexpensive. It is also desirable that the radar system and method enable existing ocean sensing radar systems to be easily and inexpensively upgraded.
The radar system and method of the present invention provides these and other advantages over the prior art.
The present invention provides a bistatic radar system and method for mapping surface currents in bodies of water that offers both expanded coverage area and stable estimates of current velocity in zones or regions where conventional backscatter radar systems have not been effective.
It is an object of the present invention to provide oceanic current information using a bistatic radar system that improves coverage area and accuracy in regions where instabilities normally preclude monitoring with conventional backscatter radar systems.
It is a further object of the present invention to reduce complexity, cost, and use of radio spectral resources by synchronizing the modulating signals of transmitters and receivers of a bistatic radar system with Global Positioning System (GPS) timing signals.
It is a yet further object of the present invention to provide a method and computer program product for transforming or converting time-delay or range, bearing, and velocity relationships used in conventional backscatter radar systems into the required current velocity maps used with elliptic/hyperbolic geomnetries of bistatic radar systems.
According to one aspect of the present invention, a bistatic radar system is provided having a number of transmitters and receivers for transmitting and receiving radar signals. Preferably, at least one transmitter is positioned in a location separate from at least one receiver, and each of the transmitting and receivers include a local oscillator locked to Global Positioning System (GPS) signals to provide the necessary coherency between the transmitters and receivers. More preferably, the transmitters and receivers are configured to detect and measure oceanic conditions, and the bistatic radar system further includes signal processing means adapted to derive information on the oceanic conditions. Oceanic conditions detected and measured by the number of transmitters and receivers can include, for example, surface current velocity vectors.
Generally, the signal processing means is adapted to determine a current velocity within a scattering cell using a Doppler shift (fD) of the radar echoes, and the following equation:       V    h    =                    f        D            ±                                    g            πλ                    ⁢          cos          ⁢                      θ            2                                              2        λ            ⁢      cos      ⁢              θ        2            
where xcex is the wavelength of the radar signals; g is the acceleration of gravity (9.806 m/s2); xcex8 is a bistatic angle between lines connecting the transmitter and the scattering cell and the receiver and the scattering cell; and Vh is the velocity component measurable from the bistatic radar system. The current velocity, Vh, is determined along a hyperbola perpendicular to an ellipse passing through the scattering cell and confocal about the transmitter and receiver, the ellipse having a constant time delay (D) equal to the measured radar echo time delay. In one embodiment, the Doppler shift (fD) is measured directly using the bistatic radar system. In another embodiment, the signal processing means is adapted to calculate a Doppler shift (fD) using the following equation:       f    D    =            ±                        g          πλ                      +          2      ⁢                        V          r                λ            
where xcex is the wavelength of the radar signals; g is the acceleration of gravity (9.806 m/s2); and Vh is a pseudo-radial current velocity derived in a pseudo backscatter stage using a computer program developed for a backscatter radar system and oceanic conditions detected and measured by the plurality of transmitters and receivers.
Preferably, the bistatic radar system is adapted to provide surface current velocity vectors that are independent of motions of waves having velocities over a Doppler spectral region substantially the same as the surface current velocity vectors. More preferably, the bistatic radar system is adapted to provide total current vectors in regions along lines joining multiple receivers.
According to another aspect of the present invention, a method is provided for mapping surface current vectors using a radar system including a number of transmitters and receivers, including a bistatic radar subsystem having at least one transmitter positioned in a location separated from at least one receiver. Generally, the method involves steps of: (i) scattering radar signals from the transmitter off ocean waves in a scattering cell to produce echoes in the receiver; (ii) determining a bearing angle (xcfx86) to the scattering cell using using a bearing determining algorithm; (iii) sampling versus time after transmission to measure radar echo time delay from the transmitter to the receiver; (iv) determining the location of the scattering cell; and (v) determinig a current velocity at the scattering cell.
Generally, the step of determining a current velocity at the scattering cell is based on the bistatic angle between lines connecting the transmitter and the scattering cell and the receiver, the Doppler shift (fD) of the radar echoes and the position of the scattering cell. As provided above, the Doppler shift can either be measured directly from the Doppler shift of a bistatic transmitterxe2x80x94receiver pair, or calculated from a hypothetical radial backscatter velocity component measured relative to one of the receivers.
The procedure for determining a position of the scattering cell includes the steps of: (i) determining a major axis (A) of an ellipse passing through the scattering cell and confocal about the transmitter and receiver, the ellipse having a constant time delay (D) equal to the measured radar echo time delay; (ii) determining a minor axis (B) of the ellipse; (iii) determining sine and cosine of an angle (xcexa8) from the scattering cell to an origin of a local coordinate system; and (iv) determining the position of the scattering cell within the local coordinate system from the sine and cosine of the angle and the major and minor axis of the ellipse.
In yet another aspect, the present invention is directed to a computer program product for use in conjunction with a computer system to accomplish steps of the above method. The computer program product includes a computer readable storage medium and a computer program mechanism embedded therein. The computer program mechanism, includes a program module that directs the computer system, to function in a specified manner, to map surface current vectors using a radar system including a number of transmitters and receivers, including a bistatic radar subsystem having at least one transmitter positioned in a location separated from at least one receiver. Generally, the computer program includes: (i) a radar control subroutine or program module; (ii) a bearing angle (xcfx86) determining subroutine or program module; (iii) an echo time delay subroutine or program module; (iv) a position determining subroutine or program module; and (v) a current velocity subroutine or program module.
Advantages of the computer program and method of the present invention include any one or all of the following:
(i) reduced complexity, cost, and use of radio spectral resources through synchronization of transmitters and receivers using GPS tuimng signals;
(ii) ability to quickly and relatively inexpensively improve the coverage area and accuracy of conventional current-mapping backscatter radar systems by deploying additional transmitters andlor receivers;
(iii) compact transmit subsystem designs, requiring no computer nor air-conditioning resources, and suitable for remote deployment where solar energy is a preferred source;
(iv) ability to radiate from vertical whip transmit antennas that can be mounted on buoys, offshore structures, or building rooftops without incurring the resulting complications of antenna pattern distortions that can affect the ability to measure bearing angles accurately; and
(v) elimination of antenna pattern distortions caused by uncontrolled rotation of an antenna mounted on a buoy or offshore structure due to wave motion.